Low-energy 17O(n,g)18O reaction within the microscopic potential model and its role for the weak r-process

The neutron radiative capture reaction $^{17}$O(n,$γ$)18O plays a pivotal role in both nuclear structure studies and astrophysical nucleosynthesis, particularly in the formation of elements during hydrostatic and explosive stellar environments. We calculated the $^{17}$O(n,$γ$)$^{18}$O cross section within the Skyrme Hartree-Fock potential model and analyzed electric dipole E1 transitions to both positive and negative-parity states below the alpha-decay threshold in $^{18}$O. Our cross sections are significantly different from the data available in commonly used libraries. We further investigate the impact of the new calculated cross section on weak r-process nucleosynthesis using large-scale reaction network calculations across a wide range of electron fractions and entropies. Our results show that the $^{17}$O(n, $γ$)$^{18}$O reaction rate significantly influences the production of first r-process peak elements, such as strontium, under specific astrophysical conditions. This study highlights the importance of accurate nuclear dat$ for light isotopes in modeling heavy-element synthesis and provides updated reaction rates for future nucleosynthesis simulations.

💡 Research Summary

The paper presents a microscopic calculation of the low‑energy neutron radiative capture reaction (^{17})O(n,γ)(^{18})O using a Skyrme Hartree‑Fock (HF) potential model and investigates its impact on the weak r‑process nucleosynthesis. The authors first construct a self‑consistent HF mean‑field for the 17 O core (spin (I=5/2^+)) plus an extra neutron. The capture is treated as an electromagnetic E1 transition from a scattering state (continuum) to a bound state of (^{18})O. The E1 matrix element is expressed in terms of a reduced single‑particle matrix element, a geometrical factor involving 6j symbols, and a radial overlap integral (I_{E1}). Spectroscopic factors (SF) extracted from the (^{17})O(d,p)(^{18})O reaction are introduced to account for missing configuration mixing.

Two widely used Skyrme parameterizations, SLy4 and SkP, are employed. For each bound configuration (both positive‑parity sd‑shell states and negative‑parity p‑shell states) a scaling factor (\lambda_b) is applied to the depth of the bound‑state potential to reproduce the experimental neutron separation energy. The resulting (\lambda_b) values are listed in Tables I–IV and show that many low‑lying states are essentially pure single‑particle configurations (λ ≈ 1), especially the 1d(_{5/2}) components, whereas p‑shell states have much smaller SF (≈ 0.3–0.5).

Partial‑wave analysis of the capture shows that the dominant contribution at astrophysical energies comes from p‑wave neutrons captured into the 2s({1/2}) bound state; transitions into the 1d({5/2}) level are roughly an order of magnitude weaker because the s‑wave lacks a centrifugal barrier. The presence of a narrow d(_{3/2}) resonance near 240 keV in the SLy4 interaction is identified, but its effect on the low‑energy cross section is minor. Sub‑threshold resonances and near‑threshold states also enhance the capture rate, consistent with the direct‑capture picture for light nuclei where the Hauser‑Feshbach statistical model is not applicable.

The calculated capture cross sections differ substantially from those tabulated in common libraries (e.g., ENDF/B‑VIII). To assess the astrophysical relevance, the authors incorporate the new rate into a large‑scale reaction network covering a wide range of electron fractions ((Y_e)) and entropies typical of core‑collapse supernovae and neutron‑star merger ejecta. The network results reveal that the revised (^{17})O(n,γ) rate can significantly modify the production of first‑peak r‑process nuclei, especially strontium (Z = 38), under conditions where the neutron‑to‑seed ratio is modest (the so‑called weak r‑process). The effect is most pronounced for moderately neutron‑rich trajectories ((Y_e\approx0.40)–0.45) and entropies of 30–80 (k_B) per baryon, where the seed nuclei are built from light isotopes and the (^{17})O capture serves as a bottleneck or accelerator for neutron flow into the iron‑group region.

In summary, the work demonstrates that (i) a microscopic Skyrme‑HF potential model provides a reliable description of low‑energy neutron capture on light nuclei, (ii) the resulting cross sections can deviate markedly from evaluated data libraries, and (iii) accurate rates for light isotopes such as (^{17})O are essential for realistic modeling of the weak r‑process and for interpreting observed variations in first‑peak r‑process abundances in metal‑poor stars and kilonova ejecta. The authors supply updated reaction rates for future nucleosynthesis simulations, highlighting the need for continued experimental and theoretical efforts on light‑nucleus capture reactions.